Metal sheets made of iron, an iron-based alloy, aluminum, an aluminum-based alloy, copper or a copper-based alloy, for instance, have so far been widely subjected to chromate treatment for rust prevention using chromates containing hexavalent chromium, among others, and, if necessary, further to coating with an organic resin for providing them with high-level corrosion resistance, fingerprint resistance, anti-scratch property, lubricity and so forth, followed by over coating with any of various coating compositions according to need.
In view of the increasing interest in environmental problems in recent years, there is a trend toward omission of the chromate treatment so far given to metal sheets. Since the chromate-treated metal sheet has high-level corrosion resistance and coating receptivity, it is anticipated that the omission of chromate treatment will cause marked reductions in such performance characteristics. Thus, it has come to be demanded that a rust preventive layer having good corrosion resistance and coating receptivity be formed by only one-step treatment with an organic resin without carrying out chromate surface treatment.
Japanese Kokai Publication 2001-164182 discloses a rust preventive coating agent comprising a water-borne resin composition obtained by reacting a water-borne polyurethane resin, a water-borne polyolefin resin, water-dispersed silica and a composition comprising a silane coupling agent and/or a hydrolyzate condensate derived therefrom together, a thiocarbonyl group-containing compound and the phosphate ion, a method for rust preventive treatment which comprises applying the coating agent, and a metal material treated for rust prevention by coating with the coating agent. However, the use of such a rust preventive coating agent having a relatively high acid value has a problem in that the coats to be obtained are poor in alkali resistance.
Japanese Kokai Publication 2001-199003 discloses an organic composite-coated steel sheet having an organic composite coat comprising a composite resin derived from specific organic resins on the surface of a zinc-coated steel sheet or aluminum-coated steel sheet. However, the formed coat is insufficient in the degree of crosslinking and, for example, it is a problem that the coat is greatly damaged in the solvent rubbing test.
Japanese Kokai Publication 2001-214283 discloses a surface-treated zinc-coated steel sheet having a coat layer formed by applying an aqueous composition comprising a metal compound, a water-soluble organic resin and an acid to the surface of the zinc-coated steel sheet. However, the water-soluble resin has a relatively high carboxyl group content and therefore the sheet is poor in alkali resistance; this is a problem.
Japanese Kokai Publication 2003-49281 discloses a surface-treated metal sheet having an organic coat formed, on at least one side of the metal sheet, from an epoxy resin and a glycoluril resin. However, the organic coat comprises no rust preventive agent and, therefore, it has a problem that the corrosion resistance is insufficient.
Japanese Kokai Publication 2003-105555 discloses a surface-treated steel sheet having a surface treatment coat formed on the surface of a zinc-coated steel sheet or aluminum-coated steel sheet, from a surface treating composition comprising (a) a water-dispersible resin and/or water-soluble resin, (b) a silane coupling agent and (c) phosphoric acid and/or a hexafluorometallic acid. However, the coat comprises a large amount of an acidic component such as phosphoric acid and, therefore, has a problem in that the alkali resistance is insufficient.